This invention relates to oil free (dry) vacuum pumps and, more particularly to such pumps having a screw rotor mechanism.
A screw pump comprising two externally threaded or vaned rotors mounted in a pump body and adapted for counter-rotation in the body with intermeshing of the rotor threads is well known. Close tolerances between the rotor threads at the points of intermeshing and with the internal surfaces of the pump body causes volumes of gas being pumped between an inlet and an outlet to be trapped between the threads of the rotors and the internal surface of the pump body and thereby urged through the pump as the rotors rotate.
Such screw pumps are potentially attractive because they can be manufactured with few working components and they have an ability to pump from a high vacuum environment at the pump inlet down to atmospheric pressure at the pump outlet.
Screw pumps are generally designed with each screw rotor being of cylindrical form overall, with the screw thread tip cross section being substantially constant along the length of the rotor. This has a disadvantage in vacuum pumps in particular that no volumetric compression is generated in use of the pump along the length of the rotor, thereby detrimentally affecting the pump's power consumption.
Screw vacuum pump s are commonly used in the semiconductor industry and, as such, need to be capable of maintaining a clean environment associated with semiconductor device processing, especially in that area of the pump--the pump inlet--closest to the semiconductor processing chamber to which the pump is attached.
A disadvantage associated with screw pumps in general is that the relatively long screw rotor length of vacuum pumps is that they need to have their rotor shafts held in bearing at each end of the shaft, i.e. including the end associated with the pump inlet. As such, the lubricants necessarily associated with these bearings may tend to leak upstream of the gas flow through the pump and thereby contaminate the semiconductor chamber to which it is attached.
The invention is concerned with overcoming such disadvantages and to provide a screw pump with improved power consumption coupled with improved lubricant containment.